A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft X-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring or based on free electron laser.
Thin transmissive EUV membranes are often required in EUV lithographic apparatus for a number of reasons. One such reason may be to protect, for example, reticles and/or lithographic components from contamination by particles (with a grain size ranging from nm to μm). Another reason may be to spectrally filter out unwanted radiation wavelengths from the generated EUV radiation.
The transmissive EUV membranes (or shortly EUV membranes) are required to be highly transparent to EUV radiation, and therefore need to be extremely thin. Typical EUV membranes have a thickness of 10 to 100 nm, to minimize absorption of EUV radiation.
EUV membranes may comprise a free-suspended (i.e. self-standing) membrane (a film) comprising a material such as polysilicon (poly-Si), produced by etching of a silicon wafer. EUV membranes may also comprise one or more layers of protective coatings (e.g. protective cap layers) on one or both surfaces to prevent EUV-induced plasma etching (for example induced by hydrogen (H, H+, H2+ and/or H3+)).
Although absorption of EUV radiation by EUV membranes may be low, it is in practice still not zero and absorption of residual EUV radiation results in an increase in temperature of the EUV membrane. Because pellicles are in vacuum, the main process for pellicle cooling is radiative heat transfer. Should the temperature of an EUV membrane exceed a damage threshold (for example, about 500 to 700° C.), damage to the EUV membrane may occur. Damage can also occur, or be amplified, when there are large temperature gradients within the EUV membrane. Where such damage is severe, the EUV membrane may break, leading to damage/contamination of an unprotected reticle or other elements of the lithographic apparatus such as mirrors, or photoresist exposure to undesired non-EUV wavelength radiation, leading to a significant manufacturing process downtime.
It is apparent that maintaining the temperature of the EUV membrane below the damage threshold, as well as minimizing temperature gradients, can increase the EUV membrane lifetime.
The reason that pellicles may fail due to heat load is that they do not absorb/emit IR radiation very well, especially for high power EUV radiation sources such as 125 Watt sources and beyond. Since thermal radiation is emitted in the IR wavelength region, a high spectral (IR) hemispherical emissivity enables a substantial heat loss for EUV membranes. It is therefore desirable to manufacture EUV pellicles which have a high spectral emissivity. Also, EUV pellicles need to be very thin if a rage amount of EUV radiation such as 90% or more is to be transmitted through an EUV membrane.